1. Field of the Invention
The present invention relates to a sealed-type remote pressure-monitoring device. More particularly, the present invention relates to a sealed-type remote device for monitoring the internal pressures of animal organs, especially, the human eyeballs, heart, brain and so on. Also, the present invention is concerned with a method for fabricating such a device.
2. Description of the Prior Art
Medicinally, the internal pressures of human organs have significantly important meanings in treating the organs. Particularly, measuring the internal pressure of the eyeball, that is, intraocular pressure is very important because higher intraocular pressures than normal may damage visual cells. Once being damaged, the visual cells are virtually impossible to recover to a normal state. In the worst case, if not suitably treated in time, the patients may lose their sight permanently.
For this reason, extensive research has been made on methods for measuring intraocular pressures, for several decades. The intraocular pressure-sensing methods developed thus far are generally divided into a physically sensing method, an optically sensing method, and a directly sensing method using a sensor.
The physically sensing method is characterized in applying physical force to the eyeball to sense the intraocular pressure. For example, an already known pressure is applied to the eyes, followed by measuring the flexure of the eyeball. Another example of this physically sensing method is to drop glass beads or metal beads on the eyeball and then to measure the height to which they bounce.
As for the optically sensing method, it takes advantage of the change in the optical refractive index or reflection of the eyeball. For example, after a medium such as air is injected to the eyes, an optical change in the eyeball is measured. Alternatively, a reflection change in the eyeball may be detected by applying a pressure to the eyeball.
The directly sensing method utilizes a sensor in measuring intraocular pressures. This method has an advantage over the above-mentioned indirect methods in that more accurate values for the intraocular pressure of the eyeball can be obtained by virtue of the sensor which is directly inserted in the eyes. In this regard, the pressure sensor inserted in the eyes transmits the information on the internal pressure of the eyeball to the outside by use of inductive coupling using an inductor.
Of the three representative intraocular pressure-sensing techniques, the directly sensing method has become predominant in recent years for its accuracy and convenience. In the physically or optically sensing method, a stimulus must be given to the eyeball whenever a measurement is taken on the intraocular pressure, imposing a significant burden on the patients who should be continually observed. What is worse, these indirect methods are inaccurate in sensing the internal pressures. From the late 1960s, many attempts have been made to measure intraocular pressures with the aid of wireless devices with advances in the semiconductor industry.
A remote sensing technique related to the invention is found in the article "Miniature Passive Pressure Transensor for Implantation in the Eye", IEEE Trans. BME. 14, pp. 74-83, 1967, yielded to C. C. Collins, which discloses a pressure-sensing device having two coiled inductors which are faced toward each other with a connection and sealed in a plastic package. In this structure, the plastic package is bent upon pressure application so that the distance between the two inductors varies. In turn, the mutual inductance and spray capacitance between the two coils also varies according to the change of the distance, giving rise to a change in the resonant frequency. Thus, by measuring this resonant frequency, the intraocular pressure can be monitored.
Another prior technique is exemplified by the article of L. Rogengren, entitled "A System for Passive Implantable Pressure Sensors", in Sensors and Actuators, A43, pp. 55-58, 1995. In the system, an inductor is wound around a pressure sensor made of two micromachined silicon substrates which are bonded and connected to each other in a hybrid manner. With a connection to an electrode drawn from the substrates, the inductor is enveloped in a resin. When pressure is applied to the system, the capacitance of the pressure sensor is changed, so that a corresponding change is brought about in the resonance frequency of the device because of the inductor coupled in parallel. Hence, the pressure applied can be evaluated by measuring the changed resonance frequency.
The conventional methods above mentioned, however, suffer from significant disadvantages. For instance, since the pressure-sensing device suggested in the article of C. C. Collins exposes important parts of the device externally, a plastic case harmless to the body is needed for envelopment. The system disclosed in the L. Rogengren's article is disadvantageous in that, because the pressure sensor is made of two silicon substrates, the Q value is lowered owing to the parasite capacitance between the two silicon substrates and to the hybrid coupling manner. In addition, another disadvantage of this system is that a resin is used to seal the outer inductor, requiring a harmless plastic envelope for the system.
In many patents are found the prior arts related to the present invention.
U.S. Pat. No. 4,922,913 discloses an intraocular pressure sensor which uses a piezo resistive pressure sensor. This sensor suffers from a fatal disadvantage in that, to measure the intraocular pressure of the eyeball, the sensor in the eyeball must be connected to an external detecting device via a wire. Owing to the wire drawn from the eye, it is highly apt to be infected with bacteria and the humors in the eyeball may flow out.
Another pressure-sensing technique is disclosed in U.S. Pat. No. 4,089,329, entitled "noninvasive continuous intraocular pressure monitor". According to this invention, a change in intraocular pressure induces a motion of a strain gauge, owing to which a resistance change is caused. This resistance change is detected by a transmitter, thereby transmitting the intraocular information to the detection device without the use of wires. However, since the pressure sensor is separated from the transmitter, the humors in the eyeball may flow out.
U.S. Pat. No. 5,005,577, entitled "Intraocular lens pressure monitoring device", is similar to the C. C. Collins' article in a sensing manner, disclosing that two coiled inductors are faced toward each other with a connection and a change is brought about in the distance therebetween in dependence on pressure, allowing the spray capacitance and mutual inductance between the coils to vary. The resonance frequency thus changed is detected to monitor the changed pressure. This device, however, has a disadvantage of being enveloped in a cumbersome plastic case. The plastic case itself is not harmful to the body.